1. Field of the Invention
The present invention relates to boronized sliding material and a method for producing the same. More particularly, the present invention relates to sintered sliding material, a part of which is boronized, and to a method for producing the same.
2. Description of Related Arts
Boronizing is widely applied to steel materials which have undergone rolling, forging, and casting, so as to improve the wear-resistance, oxidation-resistance, and corrosion-resistance thereof. While boronizing exhibits such improved properties, it has a drawback in that embrittlement occurs due to the hardness and brittleness of the borides. A very brittle layer of FeB is likely to form particularly on the treated surface. FeB readily cracks and embrittles, so that the material, on which FeB is formed, is inappropriate for use as sliding material.
The sintered material is usually used as is. The sintered material may occasionally be subjected to post-treatment, such as rolling, wire-drawing, staging, forging, rolling, sizing or coining. In coining, the sintered material is placed in a die and is rolled. Surface treatment of the sintered material is not usual.
Prior art of the surface treatment of the sintered material is now surveyed.
Material standard FN-0200-T stipulated in MplF (Metal Powder Institute Federation) specification relates to a case-hardenable material, which is characterized by addition of Ni and by a relatively high density in the range of from 7.2 to 7.6. In addition, SMF 2 stipulated in JPMA (Japan Powder Metallurgy Association) specification relates to material which is carburizable. Cu added in an amount of 3% or less makes the pores to disappear and hence creates the carburizing property.
Japanese Unexamined Patent Publication No. 60-21371 relates to a boronizing method. According to this method, a metallic container filled with Cr powder is compressed. The Cr powder is then sintered under such a condition that no pinholes are formed, and hence the sintered body has true density. Machining is then carried out to remove the container to obtain a wrought material. This wrought material having no pinholes, is then boronized. The method, therefore, is not the boronizing of sintered material.
Case-hardening or carburizing of sintered material has heretofore been known, whereby the sintered material as a whole is hardened. However, hardening a part of the surface of the sintered material, such as the inner surface of a tubular material, by boronizing has not been possible.
According to an experiment by the present inventors, the inner surface of a tubular sintered material was subjected to boronizing. The boronizing gas, which was in the generally generated amount, passed through the pores and leaked toward the outer surface of the tubular sintered material. Since boronizing in itself was impossible, the desired treated layer was evidently not formed on the inner surface. When the boronizing was carried out while generating a large amount of the boronizing gas, not only the surface but also the interior of the sintered body were boronized. In addition, a considerable amount of brittle FeB is formed on the surface of the pores which are present in the interior of the sintered body. The sintered body was therefore embrittled as a whole by the boronizing.